The answer to this can be found in the Prologue of Guns, Germs, and Steel. Specifically, it can be found towards the bottom of p. 25 in the paperback edition of the book. Diamond says that authors are often asked to summarize their books in one sentence. He then gives his one sentence summary. It is

History followed different courses for different peoples because of differences among peoples’ environments, not because of biological differences among peoples themselves.

What Diamond is saying here is that he does not agree with theories that hold that Europeans came to dominate the world because they are somehow genetically superior to native peoples in places like Australia or New Guinea. Instead, Diamond believes that Europeans came to dominate the world simply by dumb geographical luck. He believes that it was the environments in which peoples lived, and not their genetic characteristics, that made some peoples able to dominate others.